DCE is predominantly used as an intermediate to produce vinyl chloride monomer which in turn is used as starting material to produce polyvinyl chloride. The conversion of DCE into vinyl chloride monomer also produces hydrogen chloride HCl. HCl is preferably used to produce DCE by oxychlorination of ethene with HCl and oxygen. An alternative route to DCE is via the direct chlorination of ethene with chlorine. Both routes are taken in the large-scale industrial production of DCE, so the hydrogen chloride produced and the hydrogen chloride consumed balance in accordance with the following reaction equations:Cl2+C2H4→C2H4Cl2+218 kJ/molC2H4Cl2→C2H3Cl+HCl−71 kJ/molC2H4+2HCl+½O2→C2H4Cl2+H2O+238 kJ/mol
A plant complex for manufacture of vinyl chloride monomer (hereinafter called “VCM complex”) consists essentially of:                a plant for producing 1,2-dichloroethane (DCE) from ethene and chlorine (“direct chlorination”, an optional plant component); and        a plant for producing 1,2-dichloroethane from ethene, hydrogen chloride and oxygen (“oxychlorination”); and        a plant for distillative purification of 1,2-dichloroethane (production of feed DCE); and        a plant for thermally cracking the distillatively purified feed DCE into vinyl chloride and hydrogen chloride; and        a plant for distillative removal of hydrogen chloride and of unconverted 1,2-dichloroethane and also for purification of vinyl chloride monomer.        
The hydrogen chloride obtained by thermally cracking the 1,2-dichloroethane is returned into the oxychlorination plant and again reacted therein with ethene and oxygen to form DCE.
The reaction steps of direct chlorination and of oxychlorination are highly exothermic, whereas the thermal cracking of DCE into VCM and hydrogen chloride is endothermic.
The VCM complex described above can be operated in the balanced mode wherein all the DCE produced in the plant is also further processed in the VCM plant, and/or there is no need to import DCE.
In addition to the aforementioned balanced mode of operation, there are also modes/plants for producing DCE where the DCE quantity which would be manufactured in the direct chlorination step of the balanced mode is wholly or partly replaced by imported DCE. This operating mode or plant configuration is known as unbalanced among those skilled in the art.
There is a further unbalanced method of operation wherein the DCE-producing component plant produces more DCE than is consumed in the thermal cracking to VCM.
This excess DCE is subjected to distillative purification and then commercialized as “sales DCE”. The “sales-DCE” mode generally employs more columns to work up the DCE than the other modes. These additional columns represent additional heatsinks and can be operated with heat from other parts of the plant.
Numerous measures to save energy/recover heat in VCM and PVC production plants are known from the prior art. Measures of this type lead to a distinct reduction in operating costs and hence make a very substantial contribution to the economic viability of the plant. Measures of this type similarly also make a significant contribution to cutting the plant's CO2 output.
They also include measures whereby the reaction heat evolved by exothermic reaction steps is used to supply heat to heatsinks in the process. For instance, the reaction heat evolved by the oxychlorination reaction is used to generate steam which can be used, for example, to heat reactant preheaters or distillation columns.
Owing to the relatively high temperature level of the oxychlorination reaction, the generated steam is suitable for heating most of the heatsinks in the process. It will be appreciated that this steam is preferably used to supply heat to heatsinks which themselves require a relatively high temperature level.
The steam quantity generated in the oxychlorination plant is insufficient to heat all the heatsinks in a plant complex for production of VCM. Further heat recovery/energy saving options were accordingly sought.
One possibility is to use reaction heat from the direct chlorination reaction, which is obtained at a lower temperature level than that of the oxychlorination reaction. There are a multiplicity of proposals for this in the literature.
DE 32 25 732 A1, for instance, proposes using a recirculating stream of the liquid reaction medium from the direct chlorination step to heat a distillation column.
DE 31 37 513 A1 proposes using the reaction heat for space-heating purposes or for steam generation. However, there is a caveat with regard to steam generation via the reaction heat from the direct chlorination step in that the reaction temperature has to be raised for this to a value which in itself severely favors the formation of by-products, which in turn compromises the economic viability of the process. One way out would be for vaporous reaction medium from the direct chlorination reactor to be mechanically compressed and then used for heating purposes, as proposed in WO 01/21564 A1. This is disadvantageous because of the capital costs for the compressors needed as well as the energy costs for the compressing operation.
Existing proposals further include heating columns with vaporous reaction medium, as described in DE 199 16 753 C1 and WO 98/01407 A1 for example, and also simultaneously with vaporous and liquid reaction medium, as described in DE 199 53 762 A1.
Since direct chlorination plants and plant complexes for production of vinyl chloride monomer and vinyl chloride polymer are often integrated with a plant for chlor-alkali electrolysis, it has also been proposed to use the reaction heat of the direct chlorination reaction to concentrate aqueous sodium hydroxide solution, as described in DE 10 2005 044 177 A1 for example.
There are also energy-saving opportunities within the plant component dedicated to the distillative purification of 1,2-dichloroethane. This plant component within a VCM complex generally consists of a so-called dewatering column in which water as well as low boilers are removed from the DCE. Depending on the plant configuration, the plant may employ one or more further columns, for example for removing low boilers. The bottoms stream from the dewatering column is generally further purified in a so-called high-boilers column or DCE column. Furthermore, DCE removed from the product mixture of the thermal DCE cracking (so-called return DCE) is fed into the high-boilers column. Substances boiling higher than DCE are removed in the high-boilers column. The overhead product of the high-boilers column is the feed DCE for the thermal DCE cracking. The bottoms stream from the high-boilers column is usually further concentrated in a column operated under reduced pressure, i.e., a so-called vacuum column. The DCE removed in the vacuum column is admixed to the feed DCE stream from the top of the high-boilers column. The removed high boilers are sent to a workup stage.
The high-boilers column is the largest consumer of energy within the distillative DCE purification stage. In principle, the amount of heat recoverable in the direct chlorination plant is insufficient to cover the total energy requirements of this column. The missing heat has to be supplied by heating with steam. Nor is the high-boilers column vapor temperature attainable on heating the high-boilers column with the direct chlorination reaction heat sufficient to make the recovery of heat from the vapor economically viable.